1980 Porsche 924 WONT START

THE CAR IS GETTING FUEL AND I PUT NEW IGNITION SWITCH IN WHEN I TRY TO START SOME TIMES IF I PUT JUMPER WIRE FROM + SIDE OF COIL IT WILL START MABY RUN FOR 15 OR 20 SECONDS AND SOMETIMES SHUTS RIGHT BACK DOWN I WOULD LIKE YOUR HELP THANKS Doug